1. Field of the Invention
This invention provides a process for producing a membrane electrode assembly, and a fuel cell using the membrane electrode assembly produced by the process.
2. Background Art
Fuel cells are used for electrochemically oxidizing a fuel such as hydrogen or methanol within a cell to convert chemical energy of the fuel directly into electric energy which is then taken out. In fuel cells, unlike thermal electric power generation, for example, NOx and SOx are not emitted upon combustion of a fuel. Accordingly, fuel cells have drawn attention as a clean electric energy supply source.
A membrane electrode assembly (fuel cell electromotive part) in the fuel cell has a construction comprising an anode (a catalyst electrode, a fuel electrode), a proton conductive film, and a cathode (a catalyst electrode, an oxidant electrode) stacked in that order on top of each other. For each of the anode and cathode, the catalyst electrode among these elements comprises a current collector and a catalyst layer. Accordingly, the membrane electrode assembly can also be said to have a construction comprising an anode current collector, an anode catalyst layer, a proton conductive film, a cathode catalyst layer, and a cathode current collector stacked in that order on top of each other. The current collector is usually formed of a porous electroconductive material and also functions to supply a fuel or an oxidant into the catalyst layer and thus is also called “diffusion layer”. In fact, the catalyst layer does not always consist of a pure catalyst alone, and, in many cases, for example, the catalyst layer is a porous layer comprising materials constituting adjacent current collector and proton conductive film, that is, a catalytically active material, an electroconductive material, and a proton conductive material. Some catalyst layer has a construction comprising a catalytically active material supported directly on the porous electroconductive material as the current collector on its side in contact with the proton conductive film.
Direct methanol fuel cells will be described as an example. Specifically, a fuel mixture composed of methanol and water is supplied into an anode catalyst layer, and air (oxygen) is supplied into a cathode catalyst layer. In the electrodes, catalyst reactions respectively represented by chemical formulae (1) and (2) take place.Fuel electrode: CH3OH+H2O→CO2+6H++6e−  (1)Oxidant electrode: 6H++(3/2)O2+6e−→3H2O   (2)
As can be seen from the above chemical formulae, protons produced in the fuel electrode are transferred to the proton conductive film, and electrons are transferred to the anode current collector. In the oxidant electrode, a reaction takes place among the electrons supplied from the cathode current collector, the protons supplied from the proton conductive film, and oxygen, to allow current to flow across a pair of current collectors.
What is required for achieving excellent cell characteristics is to smoothly supply a suitable amount of a fuel to each of the electrodes, to cause a rapid and significant electrode catalyst reaction at a three-phase interface among the catalytically active material, the proton conductive material, and the fuel, to smoothly move electrons and protons, and to rapidly discharge the reaction product. In particular, the catalytic activity is highly important because the electric power which can be supplied is greatly influenced by the performance of the catalyst. In many cases, for both the cathode catalyst and the anode catalyst, platinum or an alloy containing platinum as a main constituent element is used from the viewpoints of the level of the activity and the demand for chemical stability. In particular, when methanol is directly used as a fuel, the adsorption of carbon monoxide as an intermediate material in the reaction on the surface of the catalyst deteriorates the catalytic activity. Accordingly, platinum is generally used as an alloy to promote a reaction of carbon monoxide with water. Alloys usable herein include an alloy of platinum with other platinum group element(s), for example, ruthenium, alloys of platinum with an element(s) other than the platinum group elements, and alloys of platinum with other platinum group element(s) and an element(s) other than the platinum group elements. In these catalysts, however, noble materials such as platinum group elements are necessary. Accordingly, even on the presumption that these materials are recycled, the total amount of these materials used should be reduced, and a high level of activity should be realized stably in a minimized amount of catalyst. From this viewpoint, a further increase in activity is also desired for catalysts comprising these platinum group elements as main constituent elements. In particular, in assembling a fuel cell comprising a plurality of membrane electrode assemblies connected in series, when there is a variation in catalytic activity for each membrane electrode assembly, the whole performance of the assembly is limited to the lowest performance in the performances possessed by the membrane electrode assemblies. To overcome this drawback, a large amount of catalyst should be used leading to a problem of an increase in the amount of noble resources used.
Methods for improving the activity of the catalyst to improve the properties of the fuel cell include a method in which current is allowed to flow from a cathode to an anode from an external power supply while supplying oxygen into a membrane electrode assembly on its cathode side and supplying a methanol fuel liquid to the anode side, and a method in which current is allowed to flow from a cathode to an anode from an external power supply while performing crossover of a large amount of methanol from the anode toward the cathode (U.S. Pat. No. 6,962,760).
In the above methods, hydrogen is evolved by electrolysis of a fuel liquid on the surface of a catalyst electrode on the anode side to reduce the surface of the catalyst and thus to improve the activity of the catalyst. In the above methods, however, when oxygen is allowed to flow toward the cathode, electrification causes an increase in potential of the cathode resulting in accelerated deterioration in the catalyst and surrounding constituent materials as a result of oxidation. The oxidation of methanol on the cathode side causes swelling of the proton conductive material by a large amount of overcrossed methanol which poses a problem that the performance of the cathode optimized for usual operation conditions is adversely affected. Further, since hydrogen gas is evolved in a bubble form on the anode side, separation of the contact interface of the catalyst and the proton conductive material or the proton conductive film sometimes takes place. Accordingly, continuation of the evolution of hydrogen for a long period of time poses a problem that the properties of the electrode are sometimes irreversibly deteriorated.
On the other hand, other method for improving the activity of the catalyst to improve the properties of the fuel cell disposed in the art comprises holding a small amount of a liquid containing catalyst particles as nanocolloid on a gold disk, drying the assembly in an inert atmosphere to prepare an electrode with catalyst particles held thereon, and electrolytically reducing the electrode in an aqueous sulfuric acid solution to improve the properties of the electrode over the properties of the electrode before the electrolytic reduction (A. Lewera et al, Electrochimica Acta, 51, 3950, (2006)). In fact, however, the electrode having catalytic activity improved by this method, when handled in the air, causes a considerably rapid deterioration in properties. Accordingly, the electrode treated by this method involves a problem that handling of the electrode in the air is difficult. Further, the following fact should be noted. In this supporting method, the substrate is gold, and the catalytically active material supported is nanoparticles of platinum group metal. Therefore, the catalytically active material is relatively strongly held. The application to a substrate having a porous structure which causes diffusion of a fuel, however, poses a problem that the holding strength is unsatisfactory.